The paper used as the filtration medium for automotive filters has been traditionally treated with phenolic resole type resins. This has been done to improve the paper's strength properties and to allow it to be pleated in an accordion-like shape and to hold the shape when the paper composite is cured. The standard phenolic resin used to treat automotive filter paper has relatively low mole ratios of formaldehyde to phenol so that good final paper properties, especially flexibility could be achieved. Higher mole ratio resins tend to result in brittle paper on curing.
The traditional method of making an automotive filter has been for the papermaker to treat a base filter sheet with an alcoholic solution of these phenolic resole resins. The treated sheet is passed through an oven to drive off the solvent and make a so-called B-stage sheet. This sheet is then shipped to the filter maker where it is pleated and put through an oven to further cure the sheet and to hold the shape of the pleats.
With the onset of greater concern for environmental quality, the filter manufacturers have requested paper suppliers to provide a formaldehyde-free or lower formaldehyde-containing impregnated sheet that holds its pleat and meets all of the requirements for oil filters. The paper suppliers have, in turn, requested resins suppliers to supply a water-based binder that provides performance at least comparable to that of the phenolic resole resins.
U.S. Pat. No. 4,623,462 discloses oil filters containing impregnated filter substrates which are impregnated with water-based binder comprising a latex containing at least 20% polymerized vinyl chloride in the latex solids, the latex being a homopolymer of a vinyl halide or a copolymer in which the vinyl halide is polymerized with other comonomers. The binder also contains 5 to 20 parts of cross-linking resin per 100 weight parts of latex solids and 5 to 20% catalyst for the cross-linking resin based on the weight of the cross-linking resin.
U.S. Pat. No. 4,999,239 discloses aqueous emulsions containing an ethylene-vinyl chloride copolymer and tetramethylol glycoluril for use as a binder composition suitable for application onto non-bonded filter paper. Filter paper impregnated with these emulsions maintained tensile strength and flexibility upon being subjected to hot oil.
U.S. Pat. No. 4,673,702; 4,714,731 and 4,716,192 disclose polyvinyl alcohol-stabilized vinyl chloride-ethylene copolymer emulsions as coatings for metal substrates.
U.S. 4,767,816 discloses an aqueous copolymer emulsion comprising a copolymer consisting essentially of vinyl chloride, ethylene and up to 10 wt. % hydroxyalkyl (meth)acrylate. The copolymer demonstrates improved solvent resistance and metal adhesion, especially to low energy films. It is suggested that the polyvinyl alcohol stabilized vinyl chloride-ethylene-hydroxyalkyl acrylate copolymer emulsions may also be used as a saturant binder for filter stock substrates.